1. Technical Field
This invention relates to a hot rolled steel sheet with a high ductivility, a high strength and a distinguished formability applicable to automobiles, industrial machinery, etc., and a process for producing the same. The term "sheet" means "sheet" or "plate" in the present specification and claims.
2. Description of the Prior Art
In order to make the automobile steel sheet lighter and ensure safety at collisions, steel sheets with a higher strength have been in keen demand. Steel sheets even with a high strength have been required to have a good formability. That is, a steel sheet must have a high strength and a good formability at the same time.
A dual phase steel composed of a ferrite phase and a martensite phase, which will be hereinafter referred to as "DP steel", has been so far proposed as a hot rolled steel sheet applicable to the fields requiring a high ductility. It is known that the DP steel has a more distinguished strength-ductility balance than a solid solution-intensified steel sheet with a high strength and a precipitation-intensified steel sheet with a high strength. However, there is such a limit to the strength-ductility balance as TS.times.T.El.ltoreq.2,000, where TS represents a tensile strength (kgf/mm.sup.2) and T.El represents a total elongation (%), and thus the DP steel cannot meet more strict requirements.
In order to overcome the limit to the strength-ductility balance, that is, to obtain TS.times.T.El&gt;2,000, it has been proposed to utilize a retained austenite phase. For example, the following processes have been proposed: a process for producing a steel sheet having a retained austenite phase, which comprises hot rolling a steel sheet at a finish temperature of Ar.sub.3 to Ar.sub.3 +50.degree. C., then maintaining the steel sheet at a temperature of 450.degree. C. to 650.degree. C. for 4 to 20 seconds, and then coiling the steel sheet at a temperature of not more than 350.degree. C. [Japanese Patent Application Kokai (Laid-open) No. 60-43425], a process for producing a steel sheet having a retained austenite phase, which comprising rolling a steel sheet at a finish temperature of 850.degree. C. or more with a total draft of 80% or more and under a high reduction with a draft of 60% or more for the last total three passes and a draft of 20% r more for the last pass, and successively cooling the steel sheet down to 300.degree. C. or less at a cooling rate of 50.degree. C./sec. or more [Japanese Patent Application Kokai (Laid-open) No. 60-165,320], etc.
However, the conventional processes requiring the maintenance of a steel sheet at 450.degree. to 650.degree. C. for 4 to 20 seconds during the cooling, the coiling at a low temperature such as not more than 350.degree. C., or the rolling under a high reduction are not operationally preferable with respect to the energy saving and productivity increase. The formability of the steel sheets obtained according to these processes is, for example, TS.times.T.El.ltoreq.2,416 and thus does not always fully satisfy the level required by users. A steel sheet with a higher TS.times.T.El value (desirably more than 2,416) and a process for producing the same with a higher productivity have been in keen demand.